The present invention relates to a novel phosphorus-modified alumina hydrogel composite and its method of manufacture. The invention also relates to the use of the composite, e.g., as a catalyst, or catalyst carrier in a hydrocarbon conversion process.
Alumina spheres useful as a carrier for a catalyst composition can be manufactured by the oil-drop method. An oil-drop process for manufacturing spherical alumina is taught in U.S. Pat. No. 2,620,314, the teachings of which are incorporated herein by reference. Spherical alumina is manufactured by the steps of commingling alumina hydrosol with a gelling agent which is hydrolyzable at an elevated temperature, dispersing the resulting mixture as droplets in a suspending medium thereby forming hydrogel particles, aging the hydrogel particles, washing with water, drying and calcining.
Phosphorus may be incorporated into catalyst carriers in various manners. Canadian Pat. No. 950,439 teaches the addition of a solution of phosphate ions to an alumina-containing hydrogel. In the specification, patentee defines the term "hydrogel" as an undried gel, precipitated hydrous oxide, or combinations thereof which are washed free of salts resulting from the gelation or precipitation reactions. Hydrogels are distinguished from sols in that the term "sol" refers to colloidal dispersion of polymeric aluminum hydroxide salts which behave as true liquids.
Similar to Canadian Pat. No. 950,439, U.S. Pat. No. 4,202,798 discloses a composite formed by a method which comprises adding phosphorus-containing compounds to an aqueous mixture of hydrous alumina. Hydrous alumina such as gibbsite, bayerite, randomite, etc., may be prepared by precipitation from an aqueous solution of a soluble aluminum salt such as aluminum chloride. The hydrous alumina is different from a sol in that the former is not a liquid colloidal suspension of aluminum hydroxyl chloride polymer and does not have true liquid properties.
Another method of utilizing phosphorus in the production of alumina supports is disclosed in U.S. Pat. No. 3,969,273. In particular, the patentee adds phosphate ions to a dried alumina gel. No significant change of nitrogen pore volume was observed.
U.S. Pat. No. 3,879,310 teaches the use of phosphate ions to stabilize pseudo-boehmitic alumina by incorporating the phosphate ion either during the precipitation of the pseudo-boehmitic alumina or by adding the phosphate ion to freshly precipitated pseudo-boehmitic alumina. The precipitation of the pseudo-boehmitic alumina is carried out by interacting a sodium aluminate solution with aqueous nitric acid. Phosphate ion is added to either of the reactants or simultaneously during the admixture of the sodium aluminate with the nitric acid. This phosphate stabilized pseudo-boehmitic alumina has X-ray diffraction peak intensity (l/l.sub.o) in the range of 6.5-6.8 Angstroms and contains a pseudo-boehmite content of at least 30% by weight.
U.S. Pat. No. 2,890,167 utilizes phosphorus in a reforming catalyst comprising a refractory oxide, halogen and a platinum group metal. Impregnation of the inorganic oxide with a solution of phosphoric acid is suggested as a convenient method of incorporating the phosphorus.
Aluminum phosphate is precipitated onto an alumina gel in U.S. Pat. No. 2,441,297 so as to improve the heat stability and mechanical strength of the catalyst base prepared in such a manner. Similarly, U.S. Pat. No. 2,349,827 teaches the addition of powdered aluminum phosphate to a washed hydrogel. In both cases, the resultant composition contains a physical mixture of alumina and aluminum phosphate.
U.S. Pat. No. 3,342,750 teaches several methods of producing aluminum phosphate gels. One method involves reacting an aqueous solution of aluminum chloride and phosphoric acid with ethylene oxide, the amount of ethylene oxide being sufficient to produce gelling to a hydrogel. This method, however, requires the extraction of the hydrogel with an organic water-soluble extracting agent. The extraction step is necessary to remove carbonaceous material from the hydrogel and increase the surface area of the dried and calcined gel.
In another method disclosed in the last-mentioned patent, dilute ammonium hydroxide must be slowly added to an aluminum chloride-phosphoric acid solution until the pH of the solution reaches about 1.0. A hydrogel is then formed by adjusting the pH of the solution to between 5 and 9 by adding a compound such as ammonium acetate or hexamethylenetetramine. Patentee's hydrogel must then be extracted with an organic water soluble extracting agent to prevent the formation of the carbonaceous materials. Patentee points out that it is essential that three preparation variables be observed to obtain high surface area aluminum phosphate gels, namely, (1) very slow addition of NH.sub.4 OH; (2) a final pH of 5-6; and (3) removal of water by extraction before drying.
The composite of U.S. Pat. No. 3,342,750, though exhibiting high surface area at the outset, is hydrothermally unstable (see column 12, lines 4-13). Furthermore, the AlPO.sub.4 gel prepared in accordance with the methods disclosed in U.S. Pat. No. 3,342,750 possesses significant catalytic cracking activity (see Example 10) coupled with a relatively small pore diameter.
U.S. Pat. No. 4,210,560 discloses a catalyst support comprising magnesia-alumina-aluminum phosphate matrix. In contrast to an "oil drop" gelation procedure, the patentee employs a precipitation procedure wherein aluminum salts, magnesium salts, and phosphoric acid are precipitated with ammonium hydroxide. Due to the specific method of precipitation, the matrix possesses an amorphous morphology.
Finally, U.S. Pat. No. 4,080,311 teaches a thermally stable composite precipitate containing aluminum phosphate and alumina having a surface area of from about 100 to 200 m.sup.2 /g. The composite disclosed, however, is an alumina-aluminum phosphate composite precipitate and not a gel. The art has recognized that gels are distinct from precipitates. (See Ware, J. C., Chemistry of the Colloidal State, Gels, Chapter XII, the teachings of which are incorporated herein.
There have been various attempts to account for and reconcile the differences between gels and precipitates. In general, however, a gel is considered to be a solidified sol with a high degree of reversibility between the two states. The gel structure is believed to be a capillary arrangement of fibrils or a "brush heap" which develops from a dispersed phase. The substance, or "fibers", constituting the dispersed phase by way of capillary attraction or adhesion, encloses the liquid phase forming the gel. The orientation-coalescence formation of a gel is an essential distinction between precipitation and gelation. In precipitation, the holding power between the suspension and water is decreased or destroyed, while in a gel this adhesion remains unimpaired. In any event, the various theories evince basic characteristic differences between gels and precipitates.
Kehl, in U.S. Pat. No. 4,080,311, employs an aluminum salt as alumina starting material and not a colloidal polymeric aluminum sol. This difference in starting material results in the formation of an alumina-aluminum phosphate composite precipitate. See column 3, lines 14-15. Kehl asserts that the precipitate is a new composition of matter possessing excellent thermal stability together with relatively high average pore radii.
It has now been discovered that a phosphorus-modified alumina hydrogel composite exhibits increased surface area and micropore volume while maintaining average pore diameter and decreasing alumina crystallite size and average bulk density. This novel composite is particularly suited for use as a catalyst support in hydrocarbon conversion catalytic processes offering higher catalyst activity and stability.